This invention relates to a feedback control method of controlling the air-fuel ratio of an air-fuel mixture being supplied to an internal combustion engine, and more particularly to a method of this kind, which is applied at a transition of the engine operation from a region other than the feeback control region to the latter region.
A fuel supply control method for an internal combustion engine, particularly a gasoline engine, has been proposed, which is adapted to determine the valve opening period of a fuel injection device for control of the fuel injection quantity, i.e. the air-fuel ratio of an air-fuel mixture being supplied to the engine, by first determining a basic value of the above valve opening period as a function of engine speed and intake pipe absolute pressure and then adding to and/or multiplying same by variables and/or coefficients indicative of operating conditions of the engine, such as engine speed, intake pipe absolute pressure, engine temperature, throttle valve opening, exhaust gas ingredient concentration (oxygen concentration), etc, by electronic computing means.
According to this proposed method, while the engine is operating in a normal operating condition, the air-fuel ratio is controlled in closed loop or feedback mode such that the valve opening period of the fuel injection device is controlled by varying the value of a coefficient in response to the output of an exhaust gas ingredient concentration detecting means which is arranged in the exhaust system of the engine, so as to attain a theoretical air/fuel ratio of a value close thereto (closed loop control), whereas while the engine is operating in one of particular operating conditions (e.g. a mixture-leaning region, a wide-open-throttle region, and a fuel-cut effecting region), the air-fuel ratio is controlled in open loop mode by the use of a mean value of values of the above coefficient applied during the preceding feedback control, together with an exclusive coefficient corresponding to the kind of the particular operating region in which the engine is then operating, thereby preventing deviation of the air-fuel ratio from a desired air-fuel ratio due to variations in the performance of various engine operating condition sensors and a system for controlling or driving the fuel injection device, etc., which are caused by machining tolerances or the like and/or due to aging changes in the performance of the sensors and the system, and also achieving required air-fuel ratios best suited for the respective particular operating conditions, to thus reduce the fuel consumption as well as improve the driveability of the engine.
However, according to this method, when a transition occurs in the engine operation from one of the above particular operating regions wherein the air-fuel ratio is controlled in open loop mode of the feedback control region, the feedback control is initiated by the use of a mean value of values of the above feedback control correction coefficient applied during the past feedback control. As a result, there exists a time lag in controlling air-fuel ratio to a desired value until the above feedback control correction coefficient assumes a value appropriate for attaining desired emission characteristics during the feedback control. Therefore, if the method is applied to an internal combustion engine having an exhaust gas purifying device, such as a threeway catalyst, until a period of time corresponding to the above time lag elapses, the amount of exhaust gas ingredient NOx can increase if the air-fuel ratio varies from a leaner value to the appropriate value, whereas the amounts of ingredients CO, UHC, etc. in the exhaust gases can increase if the air-fuel ratio varies from a richer value to the above appropriate value.